1. Field of the Invention
The invention relates to optical circulators.
2. Description of the Related Art
Optical circulators are devices having multiple optical ports and include optical structures that transfer light from an optical port n to another optical port n+1. For example, a three port optical circulator has three optical ports and an optical structure that transfers light from a first optical port to a second optical port and from the second optical port to a third optical port. Various optical structures have been proposed for optical circulators. The schematic in FIG. 13 shows one example of a typical three port optical circulator having an optical circulator unit 30, a first optical port 31 and a third optical port 33 arranged adjacent to each other on one end of the optical circulator unit 30, and a second optical port 32 disposed on the opposite end of the optical circulator.
Light enters an optical circulator at a light entry port and then propagates through the optical circulator to an associated light exit port. In a typical three port optical circulator, the first optical port 31 and the second optical port 32 are light entry ports while the second optical port 32 and the third optical port 33 are light exit ports. Thus, the second optical port 32 functions both as a light entry port, for light propagating to the third optical port 33, and a light exit port, for light emanating from the first optical port 31. Due to its relationship with both the first optical port 31 and third optical port 33, the proper alignment of the second optical port 32 with the first optical port 31 must be done in conjunction with the alignment of the third optical port 33. In addition, a positional change of one optical port correspondingly affects the other two optical ports.
The optical circulator unit 30 typically contains a series of optical elements functionally structured to pass light from the first optical port 31 to the second optical port 32 and the second optical port 32 to the third optical port 33. One example of a conventional optical circulator unit showing the optical element structure is depicted in FIG. 14, which is the one proposed in the Japanese Patent No. 2,539,563. The optical circulator unit 30 in FIG. 14 comprises optical elements arranged in a series along an optical axis Z direction, with the optical surfaces (light incidence surface and light exit surfaces) of adjacent optical elements facing each other. The structure of the typical optical circulator unit 30 in FIG. 14 specifically consists of a first birefringent crystal plate 1, a first split half wave plate 2, a first Faraday rotator 3, a second birefringent crystal plate 4, a second Faraday rotator 5, a second split half wave plate 6, and a third birefringent crystal plate 7, arranged in that order.
Another structural example of a conventional optical circulator unit is shown in FIG. 15. While the resulting optical functionality of the optical circulator unit 30 shown in FIG. 15 is the same as the one shown in FIG. 14, the number of optical elements used in the structure of the optical circulator unit 30 in FIG. 15 has been reduced. Specifically, a first split Faraday rotator 15 in FIG. 15 replaces the first split half wave plate 2 and the first Faraday rotator 3, shown in FIG. 14, and a second split Faraday rotator 16 as shown in FIG. 15 replaces the second split half wave plate 6 and the second Faraday rotator 5 shown in FIG. 14.
One problem with conventional optical circulators is uncontrolled optical system return loss due to reflecting incident light back along the same path it came from. Additionally, optical waveguide core alignment problems relating to the complexity of aligning three associated optical cores can decrease the optical circulator reliability. Optical communication systems employing optical circulators can be adversely affected by both of these problems.
The invention comprises optical circulators, methods of making optical elements for an optical circulator, methods of controlling light in an optical circulator, and optical communication systems using an optical circulator. Optical circulators are provided for controlling the back propagation of optical signals that occur within the optical circulator. In one embodiment, the invention comprises an optical circulator comprising at least three optical ports and at least one optical element having optical surfaces slanted with respect to an optical axis so as to form at least a pair of oblique optical surfaces, the relative slant of the optical surfaces such that the direction of the optical path exiting the optical element is at least substantially parallel to the direction of the optical path entering the optical element.
In another embodiment, the invention comprises an optical circulator having at least three optical ports and configured to route light input at port n to port n+1 comprising at least one optical element of a non-rectangular parallelepiped shape.
In yet another embodiment, the invention comprises an optical circulator comprising at least three optical signal ports and a substantially parallelepiped shaped optical element disposed so that its optical surfaces are non-perpendicular to an optical axis, whereby at least some incident light is reflected in a direction non-parallel to its incident direction.
In another embodiment, the invention comprises an optical circulator comprising a first lens and a second lens, a first optical waveguide having a first optical port and a third optical waveguide having a third optical port arranged adjacent to each other and facing the first lens, positioned with either the first optical waveguide and the first optical port or the third optical waveguide and the third optical port aligned along the central axis of said first lens, a second optical waveguide having a second optical port disposed facing second lens and aligned along the central axis of the second lens, and a prism disposed adjacent to the first lens adjusting the direction of light emanating from the first optical port and the direction of light propagating to the third optical port so that the optical path of light emanating from the first optical port is parallel to the optical path of light propagating to the third optical port.
In a further embodiment, the invention comprises an optical circulator comprising at least three optical signal ports, at least one optical element having a pair parallel optical surfaces, the optical surfaces having an oblique relative slant with respect to an optical axis, a first lens and a second lens, a first optical waveguide having a first optical port and a third optical waveguide having a third optical port arranged adjacent to each other so that the first optical port and the third optical port face the first lens and positioned with either the first optical waveguide and the first optical port or the third waveguide and the third optical port at least substantially aligned along the central axis of the first lens, and a second optical waveguide having a second optical port disposed facing the second lens and at least substantially aligned along the central axis of the second lens. The optical circulator further comprises a light path adjusting optical element intersecting the light path passing through the first optical port and the light path passing through the third optical port and adjusting the direction of the light paths so the light passing through the first optical port and the light passing through the third optical port is at least substantially parallel to each other, and an optical offset element configured to produce a parallel shift in the optical path of light propagating through the optical offset element.
The invention also comprises methods of making an optical circulator. In one embodiment the invention comprises a method for cutting optical material to form the oblique optical surfaces. The method includes cutting an optical material along a first plane intersecting the lateral sides parallel to the optical material""s longitudinal axis where the first plane is slanted to be oblique with respect to the longitudinal axis, cutting the optical material along a second plane at least substantially parallel to the first plane, and polishing the cut surfaces of the optical material so as to form optical surfaces. This method further comprises arranging the optical material in a structure with other optical elements so as to establish a first optical path that optically connects a first optical port and a second optical port, and also establishes a second optical path that optically connects a second optical port and a third optical port, where the optical surfaces are positioned to be oblique with respect to the direction of the incident optical paths.
In another embodiment of the invention, the method includes polishing a first surface forming a lateral side of an optical prism to form a first optical surface where the first surface is slanted to form an oblique angle with the top surface of said optical prism, polishing a second surface forming a lateral side of the optical prism to form a second optical surface where the second surface is slanted in a direction at least substantially parallel to the first surface, and cutting the optical prism along a plane intersecting the first optical surface and the second optical surface and perpendicular to the top surface of the optical prism. This method further comprises arranging the optical prism in a structure with other optical elements so as to establish a first optical path that optically connects a first optical port and a second optical port, and also establishes a second optical path that optically connects a second optical port and a third optical port, where the optical surfaces of the optical prism are positioned to be oblique with respect to the direction of the incident optical paths.
Methods for transferring light through an optical circulator are provided in another embodiment of the invention. In one embodiment of the invention, a method is provided for transferring an optical signal along an optical path in an optical circulator between an input optical port and an output optical port to control the back propagation of light along the optical path. This method comprises passing an optical signal through a first oblique optical surface and subsequently passing the optical signal through a second oblique optical surface at least substantially parallel to said first oblique optical surface.
In another embodiment of the invention, a method of propagating light through an optical circulator so as to minimize the size of the optical circulator and increase reliability is provided. This method comprises passing a first optical signal from a first optical port straight through the central axis of a first lens, through an optical circulator unit and straight through the central axis of a second lens into a second optical port, passing a second optical signal from the second port straight through the central axis of a second lens, through an optical circulator unit and through a prism, refracting the second optical signal as it passes through the prism, passing the second optical signal through the first lens, refracting the second optical signal as it passes through the first lens so it is parallel in direction to the direction of the first optical signal and passing the second optical signal into a third optical port adjacent to the first optical port.
Optical communication systems utilizing the present invention are also provided. According to one embodiment of the invention, this system comprises optical fiber, at least one optical signal emitter, and at least one optical signal receiver. The system further comprises an optical circulator comprising at least one optical element having optical surfaces slanted with respect to an optical axis so as to form at least a pair of oblique optical surfaces, the relative slant of the optical surfaces such that the optical path direction exiting said optical element is at least substantially parallel to the optical path direction entering the optical element.
In another embodiment of the invention, the optical communication system comprises optical fiber, at least one optical signal emitter and at least one optical signal receiver. The system further comprises an optical circulator comprising a first lens and a second lens, a first optical port and a third optical port disposed adjacent to each other, the first optical port and the third optical port facing the first lens and disposed with either the first optical port or the third optical port aligned along the central axis of the first lens, a second optical port disposed facing the second lens and aligned along the central axis of the second lens, and a light path adjusting optical element intersecting the light paths propagating through the first optical port and the third optical port, adjusting the light paths so the light propagating through the first optical port and the third optical port are at least substantially parallel to each other.